The force acting perpendicularly per unit area is called pressure. The SI unit of pressure is N/m2 or Pascal (Pa).
One Pascal pressure:
The pressure exerted in 1m area when 1 N of force is applied is called one Pascal pressure.
The force exerted by liquid on per unit area of the container is called liquid pressure.
Properties of liquid pressure:
i) Liquid pressure increases with increasing depth.
ii) Liquid pressure does not depend upon the volume of it.
iii) Liquid pressure is independent of the shape of the vessel in which it is kept.
iv) Pressure applied on a liquid is transmitted equally in all direction.
Pascal’s law states that, “the pressure is equally transmitted perpendicularly to all sides as pressure is applied at a place on a liquid contained in a closed container.”
Applications of Pascal’s Law:
Pascal’s law is applicable for the construction of hydraulic press, hydraulic brake, hydraulic lift, hydraulic cranes and other hydraulic machines.
7 Principle of hydraulic press:
The principle of hydraulic machine states that, “a small force applied on a smaller piston is transmitted to produce a large force on the bigger piston.”
Reasons for using liquid in hydraulic press:
i) Liquid transnits pressure equally and perpendicularly in all directions.
ii) Any liquid can not be compressed. i.e. liquid is almost incompressible.
Uses of hydraulic press:
A hydraulic press is used mainly for the following purposes:
i) For pressing cotton bales and good like quilts, books, paper, metal sheets, etc.
ii) For extracting the juice from sugarcane, sugar beet, fruits, etc.
iii) For squeezing oil out of linseed and cotton seeds.
The upward force exerted by a liquid on a body which is immersed in the liquid is known as the upthrust. It is measured in Newton (N).
Upthrust (U) = W1-W2 Where,
W1 = weight of an object in air
W2 = weight of an object when immersed in the liquid.
The upthrust of a liquid depends on:
i) The density of the liquid in which the body is immersed (U ∝ d).
ii) The size or volume of the body immersed in a liquid (U ∝ V).
iii) The value of acceleration due to gravity (U ∝ g).
The forees produced when a body is float or immersed in a liquid:
i) Upthrust (acting vertically upward)
ii) Weight of a body (acting vertically downward)
The mass per unit volume of a substance is called its density. SI unit of density is kg/m3 and CGS unit is g/cm3.
Density (d) = volume(v)/mass(m)
Relative density of a substance is defined as the ratio of the mass of a certain volume of the substance to the mass of the equal volume of water at 4°C.
Relative density =
(Mass of certain volume of the substance) / (Mass of the same volume of water at 4°C)
Relative density has no unit.
The relation between density of liquid and its upthrust:
Upthrust is directly proportional to the density of the liquid.
i.e. U = Vdg
“When a body is partially or wholly immersed in a liquid, it experiences an upthrust which is equal to the weight of the liquid displaced by it.”
So, Upthrust = Weight of displaced liquid
Applications of Archimedes’ Principle:
Archimedes’ principle is used to deign :
i) The ships and submarines.
ii) The hydrometers to find the densities of liquids.
iii) The lactometers to test the purity of milk.
iv) The hot air ballons.
v) It is used in determining the relative density of a substance.
Law of floatation:
An object floating in the liquid displaces the liquid equal to its own weight.
Hence, Weight of floating body = Weight of displaced liquid.
The instrument, which is used to measure relative density of a substance and density of the liquid is called hydrometer.
Types of Hydrometer:
Hydrometer is of two types,
i) Constant immersion hydrometer
ii) Constant weight hydrometer
The hydrometer which is constricted to measure the density of only the milk is called Lactometer.
Functions of hydrometer:
i) To measure the relative deasity of a substance.
ii) To measure the density or purity of milk.
iii) To measure the density of liquid.
Conditions for floatation of an object:
i) If density of a substance is less than that of the liquid.
ii) If the weight of the displaced water is equal to weight of an object.
iii) If the density of liquid and substance are equal, the substance will neither sink nor float. It will stay in equilibrium position.
The surface of earth is surrounded by a layer of air from all sides. This layer of air is called atmosphere. The region of air, which surrounds the earth is called atmosphere. The atmosphere covers water and land of the earth in the form of canopy.
The thrust exerted per unit arca on the earth surface due to column of air, is called the atmospheric pressure on the surface of the earth.
The atmospherie pressure at sea level is called standard atmospherie pressure which is 101300 N/m2 or 760 mmHg. The atmospherie pressure at the top of the Mount Everest is 3x104N/m2. The atmospherie pressure decreases as we go from sea level. Therefore, it is less at the top of Mr.Everest than Chitwan, Nepalgunj and Bhairahawa. Due to the difference in air pressure at different altitudes, the air blows from place to place.
Instruments used to measure air pressure:
a) Manometer : A manometer is an instrment used for measuring the pressure exerted by gas.
b) Mercury barometer : A mercury barometer is an instrument used for measuring the atmospherie pressure. The barometer used in the aeroplane or the mountain climbers are called Aneroid barometer. It is also called Altimeter.
c) Pressure gauge: Used for measuring the air pressure in the tubes of the wheels of vehieles such as truck, bus, car, motorcycle, etc.
Types of Barometers:
The following three types of barometers are commonly used:
i) Mercury barometer
ii) Fortin’s barometer
iii) Aneroid barometer
Uses of Barometer:
A barometer is used for the following purposes :
i) To measure the atmospheric pressure at a place.
ii) For weather forecasting.
iii) As an altimeter to measure the height. An altimeter is an aneroid barometer, but it is used in aircraft to measure its altitude.
Water cannot be used instead of mercury in a barometer because of the following reasons:
i) The density of water is less than that of mercury. Therefore, if water is used in a barometer, the height of the tube should be about 11m. It makes the barometer inconvenient to handle.
ii) Water is sticky to the walls of a barometer.
iii) Water is transparent, thus it is not clearly seen in the column.
iv) Water vaporizes easily.
Some instruments based on atmospheric pressure:
ii) Air Pump
iii) Water Pump
Importance of atmospheric pressure:
i) Atmospheric pressure balances the pressure in and out of our body. So, we are alive. In the absence of atmospheric pressure we can not survive.
ii) Due to atmospheric pressure ink can be filled in pens.
iii) Due to atmospheric pressure a syringe can work. i.e. medicine can be filled in a syringe.
iv) Due to atmospheric pressure water pumps work. i.e. It is important to lift the water by using a water pump.
v) Due to atmospheric pressure we can suck cold drinks and juices like fruit, cocacola, etc. from their containers.
vi) Due to atmospheric pressure air pumps can fill air tubes of ball and different vehicles.
vii) Due to the difference in pressure at different altitudes, the air blows from place to place. So, it causes wind and it also helps to rain.